Image: JPL Cassini
The surface of Europa, one of Jupiter's many moons. The surface of Europa is water ice. The lines come from cracks caused by Jupiter's tidal forces. Scientists think there's a liquid ocean underneath the ice. As a result, Europa is currently the best candidate for life elsewhere in the solar system.
Twenty five years ago, it was generally agreed that there was no possibility of life in the solar system outside Earth. However, three things have changed. First, we know far more about locations like Europa than previously, mainly due to Voyager and Cassini. Second, a more general acceptance of the idea that the building blocks of life formed not on Earth, but in space. This is Fred Hoyle's famous theory that life came via raining comets. Life was, rather than a completely random process, an accident waiting to happen. We should expect to see life anywhere with a combination of comets (everywhere in our solar system) and an environment conducive to life (possibly Europa, Titan and Mars at the least). Last is the dawning awareness among scientists of the success of bacteria. They are by far the most evolved and prolific life form on our planet. The only place scientists have looked and not found bacteria on earth is; nowhere. They've found bacteria everywhere they've looked. Including under a kilometer of sediment at the bottom of the ocean. If bacteria can live there, they could live in a liquid ocean under the Europa ice cap, if such an ocean exists. It will take many years before we can have a look. In the meantime, we have these marvelous and tantalizing pictures.